1. Field of the Invention
The invention pertains to control circuits for motors and in particular to a control circuit for a polyphase stepping motor in which sequence control signals are superimposed on the motor supply line.
2. Description of the Prior Art
A standard two phase stepping motor requires three to five (or more) leads connected to its power drive which, in turn, are connected to a switching logic, which logic may be in, on or in the vicinity of the stepping motor. The conventional drive circuits for stepping motors consist of appropriate motor phase driving switches, power input, trigger input, direction control, and sequence logic. The phase drive outputs must be sequenced in a defined order for proper motor operation. To change the running direction of the motor, the sequence is reversed. A trigger input signal to the drive logic advances the sequence one step at a time, and a direction input signal determines the direction of rotation. For a typical two phase stepping motor, a four step sequence is used to complete one electrical cycle. For the sequence logic, commonly a shift left/shift right register is used and an input signal is used to control the direction of shifting.
The problem with the prior art control circuits described above is that they require three to five, or even more leads connected to the power drive which, in turn, are connected to the switching logic. The number of wires and the number of connections required is excessive for the trend toward microprocessor based control systems, many of which use only a two wire bus.
U.S. Pat. No. 4,341,982 discloses a system for simultaneously and independently controlling a plurality of motors. The system operates by transmission of duty cycle modulated selected frequencies superimposed on a DC track voltage on a common power line servicing the motors. This two wire system is significantly different in purpose and construction from the present invention.
Japanese Kokai No. 58-54897 discloses a driving device for pulse motors which decreases the number of signal cables by transmitting signals having differing pulse widths on the same signal wire, discriminating between signals in response to pulse width at the receiving circuit side for driving and controlling a pulse motor.